Individual differences both in rate and pattern of aging are generally acknowledged to be great. Understanding the origins of this variability is of fundamental importance for basic scientific understanding of aging processes. This basic knowledge may also lay the groundwork for eventual application in the prediction of differential decline in functional capacity and utilization of health and care services, and in informing the search for rational therapeutic and preventive interventions. Recent research has just begun to explore the genetic and environmental bases of individual differences in aging by the study of monozygotic (identical) and dizygotic (fraternal) twins. However, these studies are principally concerned with the "young-old." Practically nothing is yet known in this regard concerning the "old-old," those individuals 80 years of age and older, who comprise the most rapidly growing segment of the population, with the highest incidence of decline in health and competence, and with the greatest need for medical care. This proposal is for support of a study of octogenarian and nonagenarian Swedish twins on measures of physical health and functioning, cognitive functioning, interpersonal relationships, life satisfaction, personality, and mental health. Analyses will estimate the relative contributions of environmental and genetic factors to the variability in these measures. All cooperating Swedish twins of the appropriate ages, estimated to number about 375 pairs, will be studied. Interpretation of the study will be enriched by the availability of data from the twin study of the "young-old" and from singleton studies of Swedish single-born octogenarians.